I'm looking, I'm looking.
Allopreening in owls. This is pertinent:
Usually, the bird that initiated allopreening indicated its intent by staring at the other bird and uttering low cooing or whistling calls. If the other bird was receptive, it usually stared back, sometimes giving low cooing calls. After this brief solicitation exchange, one bird would fly or walk to a position beside the other (if it was not already in this position), where it would lean over and begin to preen the other's head. Typically, allopreening birds perched side by side, facing in the same direction (Fig. 1). Both birds partially or entirely closed their eyelids and nictitating membranes while allopreening, a behavior also described for the Oilbird (Steatornis caripensis) (Harrison 1965).Well, dearies (English novels again), that's in owls, and bonding pairs. Lucy will nip Charlie, much as my mother would bonk my head with a hairbrush if I wouldn't sit still.
Preening was concentrated around the facial area, the top of the head, and the side of the head facing the preener. The recipient usually moved its head, as if to facilitate preening in whatever area was being preened. Most frequently, this consisted of lowering the head to expose the nape and top of the head or turning the head slightly toward the preener to expose the facial area.
Although most bouts began with one bird preening the other, preening usually became reciprocal or simultaneous as bouts proceeded. One bird would preen the other for a period, then roles would be reversed, often several times in a single bout. Simultaneous allopreening (both birds allopreening at the same time) occurred most frequently during periods of a few seconds when birds were changing roles as reciprocal allopreeners.
While allopreening, owls frequently made vocal cooing or whistling sounds that were just barely audible. Infrequently, a short staccato series of chittering notes was also given.
Both sexes initiated allopreening bouts, but we did not record which sex initiated them most frequently. Bouts lasted from only a few seconds (infrequently) to several minutes and usually ended when one or both birds seemed to lose interest in allopreening and went to sleep or began to autopreen. When one bird wished to terminate allopreening but the other bird persisted, the former usually sidled (or flew) away.
The mechanics of allopreening were similar to autopreening; the preener would mandibulate or nibble the feathers of the other bird, occasionally sliding one or more feathers between its mandibles with a gentle vibrating motion. The principal dif- ferences between allopreening and autopreening were that allopreening motions were more rapid, and no attempt was made to preen a particular area or feather thoroughly. The rapidity of allopreening motions left the impression that owls were "running their bills through each others'... feathers," as described by Miller (1974). We never observed any instance in which owls jabbed or tugged at each other in an aggressive manner while allopreening. Paragraphs mine -- Janis
Charlie approaches Lucy as a juvenile to an adult bird (aka Mama). He never offers to preen her, and would likely be out of line if he tried.
I'm interested in the "whistling sound that is barely audible."